Because the blade of a jigsaw is only supported at one end, it can sometimes bend and wander off course, As a result, it can be difficult to effectively cut a straight line with the tool.
Using a straight edge when making linear cuts will give you an edge against which you can guide your jigsaw.
Parallel guides
Parallel guides (sometimes called side fences) are a type of straight cutting guide that allows you to make cuts parallel to the edge of the material.
The guide rail is fitted to the baseplate of the jigsaw and keeps the tool (and the blade) at a set distance from the edge of the workpiece during cutting.